A central feature in all models and theories of muscle contraction is the movement of myosin cross-bridges. We propose to investigate the extent and the nature of this movement, and in particular, the involvement of the DTNB light chains and charge interactions in regulating the disposition of the cross-bridges. In these studies we will compare phosphorylated and unphosphorylated myosin employing proteolytic digestions and crosslinking methods. The precise location of the DTNB light chains on myosin is still uncertain, although they appear to bridge the S-1 and S-2 fragments of the molecule. We plan to test the possible direct interaction between the light chains and the S-2 region of myosin. Isolated and purified S-2 fragments will be combined with the DTNB light chains under a variety of experimental conditions and their binding interaction will be monitored by spectral and hydrodynamic techniques. The formation of the thick filaments of the muscle cell will be studied employing the newly discovered myosin minifilaments. The minifilaments can grow into thick filaments. These studies will combine electron microscopy with hydrodynamic techniques. In order to develop a better understanding of the active site of myosin we will contine our attempts to affinity label the ATP site (using p-%-fluorosulfonylbenzoyladenosine) and will investigate the "trapping" and the release of the "trapped" ADP from the nucleotide site.